Rotary electrical machines driven by variable speed systems or inverters such as permanent magnet synchronous machines have increased for energy saving of electric equipment and the like. In these rotary electric machines, there is a case where a common mode voltage generated by an inverter is electrostatically coupled between a coil and a rotor to generate a potential difference (hereinafter referred to as an “axial voltage”) between inner and outer rings of a bearing, which is a problematic. This is because an excessive axial voltage leads to insulation breakdown of a lubricating oil inside the bearing and causes electrolytic corrosion of the bearing due to a current (for example, PTL 1).
PTL 1 discloses a technique that performs shielding between a coil and a rotor to reduce an axial voltage. To be specific, PTL 1 discloses a radial gap rotary electric machine which is configured such that an insulating layer is provided on the entire stator surface facing a stator core and a rotor of a coil, and a conductive portion and an insulating portion are alternately formed on a surface of the insulating layer in a direction perpendicular to flow of a magnetic flux of the stator core. That is, the conductive portion is electrically connected to the core which is set to a ground potential. As a result, a large eddy current is not generated in the conductive portion, and it is possible to obtain the shielding between the coil and the rotor and to significantly reduce the electrostatic capacitance.
The above-described radial gap rotary electric machine is a structure of the current mainstream. In recent years, however, axial air gap rotary electric machines in which a stator and a rotor face each other with a predetermined air gap therebetween in a rotation axis direction have been also actively developed. In the axial air gap rotary electric machine, an opposing area between the stator and the rotor per unit size increases in proportion to about the square of a diameter as the diameter is enlarged, so that it is easy to achieve high output density and high efficiency. Therefore, the present rotary electrical machine has a structure suitable for thinning and flattening. Meanwhile, a plurality of cores are arranged in an electrically-insulated state, and the opposing area between the coil and the rotor is large, which provides a structure in which the axial voltage is likely to increase.
PTL 2 discloses a technique that reduces an axial voltage focusing on a structure peculiar to an axial air gap, such as a grounded structure of a plurality of cores, and a shield structure between a coil and a rotor, and a shaft. To be specific, PTL 2 discloses a configuration in which an end of an iron core protrudes from a bobbin around which a coil is wound, and an outer circumferential surface of the protruding iron core and an inner circumferential surface of a housing are electrically conducted via a conductive member to ground the iron core. In addition, PTL 2 discloses a configuration in which shielding between a shaft and a coil is obtained by arranging a tubular conductive member between a center portion on a rotating shaft side of a stator having an annular shape and the shaft and electrically conducting the conductive member and a housing.